ConstantFolding.cpp revision 03dd25ca964813c8b9fe14479443b9c21fb92c55
1//===-- ConstantFolding.cpp - Analyze constant folding possibilities ------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This family of functions determines the possibility of performing constant 11// folding. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/Analysis/ConstantFolding.h" 16#include "llvm/Constants.h" 17#include "llvm/DerivedTypes.h" 18#include "llvm/Function.h" 19#include "llvm/Instructions.h" 20#include "llvm/Intrinsics.h" 21#include "llvm/ADT/SmallVector.h" 22#include "llvm/Target/TargetData.h" 23#include "llvm/Support/GetElementPtrTypeIterator.h" 24#include "llvm/Support/MathExtras.h" 25#include <cerrno> 26#include <cmath> 27using namespace llvm; 28 29//===----------------------------------------------------------------------===// 30// Constant Folding internal helper functions 31//===----------------------------------------------------------------------===// 32 33/// IsConstantOffsetFromGlobal - If this constant is actually a constant offset 34/// from a global, return the global and the constant. Because of 35/// constantexprs, this function is recursive. 36static bool IsConstantOffsetFromGlobal(Constant *C, GlobalValue *&GV, 37 int64_t &Offset, const TargetData &TD) { 38 // Trivial case, constant is the global. 39 if ((GV = dyn_cast<GlobalValue>(C))) { 40 Offset = 0; 41 return true; 42 } 43 44 // Otherwise, if this isn't a constant expr, bail out. 45 ConstantExpr *CE = dyn_cast<ConstantExpr>(C); 46 if (!CE) return false; 47 48 // Look through ptr->int and ptr->ptr casts. 49 if (CE->getOpcode() == Instruction::PtrToInt || 50 CE->getOpcode() == Instruction::BitCast) 51 return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD); 52 53 // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5) 54 if (CE->getOpcode() == Instruction::GetElementPtr) { 55 // Cannot compute this if the element type of the pointer is missing size 56 // info. 57 if (!cast<PointerType>(CE->getOperand(0)->getType())->getElementType()->isSized()) 58 return false; 59 60 // If the base isn't a global+constant, we aren't either. 61 if (!IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD)) 62 return false; 63 64 // Otherwise, add any offset that our operands provide. 65 gep_type_iterator GTI = gep_type_begin(CE); 66 for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i, ++GTI) { 67 ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(i)); 68 if (!CI) return false; // Index isn't a simple constant? 69 if (CI->getZExtValue() == 0) continue; // Not adding anything. 70 71 if (const StructType *ST = dyn_cast<StructType>(*GTI)) { 72 // N = N + Offset 73 Offset += TD.getStructLayout(ST)->MemberOffsets[CI->getZExtValue()]; 74 } else { 75 const SequentialType *ST = cast<SequentialType>(*GTI); 76 Offset += TD.getTypeSize(ST->getElementType())*CI->getSExtValue(); 77 } 78 } 79 return true; 80 } 81 82 return false; 83} 84 85 86/// SymbolicallyEvaluateBinop - One of Op0/Op1 is a constant expression. 87/// Attempt to symbolically evaluate the result of a binary operator merging 88/// these together. If target data info is available, it is provided as TD, 89/// otherwise TD is null. 90static Constant *SymbolicallyEvaluateBinop(unsigned Opc, Constant *Op0, 91 Constant *Op1, const TargetData *TD){ 92 // SROA 93 94 // Fold (and 0xffffffff00000000, (shl x, 32)) -> shl. 95 // Fold (lshr (or X, Y), 32) -> (lshr [X/Y], 32) if one doesn't contribute 96 // bits. 97 98 99 // If the constant expr is something like &A[123] - &A[4].f, fold this into a 100 // constant. This happens frequently when iterating over a global array. 101 if (Opc == Instruction::Sub && TD) { 102 GlobalValue *GV1, *GV2; 103 int64_t Offs1, Offs2; 104 105 if (IsConstantOffsetFromGlobal(Op0, GV1, Offs1, *TD)) 106 if (IsConstantOffsetFromGlobal(Op1, GV2, Offs2, *TD) && 107 GV1 == GV2) { 108 // (&GV+C1) - (&GV+C2) -> C1-C2, pointer arithmetic cannot overflow. 109 return ConstantInt::get(Op0->getType(), Offs1-Offs2); 110 } 111 } 112 113 // TODO: Fold icmp setne/seteq as well. 114 return 0; 115} 116 117/// SymbolicallyEvaluateGEP - If we can symbolically evaluate the specified GEP 118/// constant expression, do so. 119static Constant *SymbolicallyEvaluateGEP(Constant** Ops, unsigned NumOps, 120 const Type *ResultTy, 121 const TargetData *TD) { 122 Constant *Ptr = Ops[0]; 123 if (!cast<PointerType>(Ptr->getType())->getElementType()->isSized()) 124 return 0; 125 126 if (TD && Ptr->isNullValue()) { 127 // If this is a constant expr gep that is effectively computing an 128 // "offsetof", fold it into 'cast int Size to T*' instead of 'gep 0, 0, 12' 129 bool isFoldableGEP = true; 130 for (unsigned i = 1; i != NumOps; ++i) 131 if (!isa<ConstantInt>(Ops[i])) { 132 isFoldableGEP = false; 133 break; 134 } 135 if (isFoldableGEP) { 136 std::vector<Value*> NewOps(Ops+1, Ops+NumOps); 137 uint64_t Offset = TD->getIndexedOffset(Ptr->getType(), NewOps); 138 Constant *C = ConstantInt::get(TD->getIntPtrType(), Offset); 139 return ConstantExpr::getIntToPtr(C, ResultTy); 140 } 141 } 142 143 return 0; 144} 145 146 147//===----------------------------------------------------------------------===// 148// Constant Folding public APIs 149//===----------------------------------------------------------------------===// 150 151 152/// ConstantFoldInstruction - Attempt to constant fold the specified 153/// instruction. If successful, the constant result is returned, if not, null 154/// is returned. Note that this function can only fail when attempting to fold 155/// instructions like loads and stores, which have no constant expression form. 156/// 157Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) { 158 if (PHINode *PN = dyn_cast<PHINode>(I)) { 159 if (PN->getNumIncomingValues() == 0) 160 return Constant::getNullValue(PN->getType()); 161 162 Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0)); 163 if (Result == 0) return 0; 164 165 // Handle PHI nodes specially here... 166 for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) 167 if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN) 168 return 0; // Not all the same incoming constants... 169 170 // If we reach here, all incoming values are the same constant. 171 return Result; 172 } 173 174 // Scan the operand list, checking to see if they are all constants, if so, 175 // hand off to ConstantFoldInstOperands. 176 SmallVector<Constant*, 8> Ops; 177 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 178 if (Constant *Op = dyn_cast<Constant>(I->getOperand(i))) 179 Ops.push_back(Op); 180 else 181 return 0; // All operands not constant! 182 183 return ConstantFoldInstOperands(I, &Ops[0], Ops.size(), TD); 184} 185 186/// ConstantFoldInstOperands - Attempt to constant fold an instruction with the 187/// specified opcode and operands. If successful, the constant result is 188/// returned, if not, null is returned. Note that this function can fail when 189/// attempting to fold instructions like loads and stores, which have no 190/// constant expression form. 191/// 192Constant *llvm::ConstantFoldInstOperands(const Instruction* I, 193 Constant** Ops, unsigned NumOps, 194 const TargetData *TD) { 195 unsigned Opc = I->getOpcode(); 196 const Type *DestTy = I->getType(); 197 198 // Handle easy binops first. 199 if (isa<BinaryOperator>(I)) { 200 if (isa<ConstantExpr>(Ops[0]) || isa<ConstantExpr>(Ops[1])) 201 if (Constant *C = SymbolicallyEvaluateBinop(I->getOpcode(), Ops[0], 202 Ops[1], TD)) 203 return C; 204 205 return ConstantExpr::get(Opc, Ops[0], Ops[1]); 206 } 207 208 switch (Opc) { 209 default: return 0; 210 case Instruction::Call: 211 if (Function *F = dyn_cast<Function>(Ops[0])) 212 if (canConstantFoldCallTo(F)) 213 return ConstantFoldCall(F, Ops+1, NumOps); 214 return 0; 215 case Instruction::ICmp: 216 case Instruction::FCmp: 217 return ConstantExpr::getCompare(cast<CmpInst>(I)->getPredicate(), Ops[0], 218 Ops[1]); 219 case Instruction::Shl: 220 case Instruction::LShr: 221 case Instruction::AShr: 222 return ConstantExpr::get(Opc, Ops[0], Ops[1]); 223 case Instruction::Trunc: 224 case Instruction::ZExt: 225 case Instruction::SExt: 226 case Instruction::FPTrunc: 227 case Instruction::FPExt: 228 case Instruction::UIToFP: 229 case Instruction::SIToFP: 230 case Instruction::FPToUI: 231 case Instruction::FPToSI: 232 case Instruction::PtrToInt: 233 case Instruction::IntToPtr: 234 case Instruction::BitCast: 235 return ConstantExpr::getCast(Opc, Ops[0], DestTy); 236 case Instruction::Select: 237 return ConstantExpr::getSelect(Ops[0], Ops[1], Ops[2]); 238 case Instruction::ExtractElement: 239 return ConstantExpr::getExtractElement(Ops[0], Ops[1]); 240 case Instruction::InsertElement: 241 return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2]); 242 case Instruction::ShuffleVector: 243 return ConstantExpr::getShuffleVector(Ops[0], Ops[1], Ops[2]); 244 case Instruction::GetElementPtr: 245 if (Constant *C = SymbolicallyEvaluateGEP(Ops, NumOps, I->getType(), TD)) 246 return C; 247 248 return ConstantExpr::getGetElementPtr(Ops[0], 249 std::vector<Constant*>(Ops+1, 250 Ops+NumOps)); 251 } 252} 253 254/// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a 255/// getelementptr constantexpr, return the constant value being addressed by the 256/// constant expression, or null if something is funny and we can't decide. 257Constant *llvm::ConstantFoldLoadThroughGEPConstantExpr(Constant *C, 258 ConstantExpr *CE) { 259 if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType())) 260 return 0; // Do not allow stepping over the value! 261 262 // Loop over all of the operands, tracking down which value we are 263 // addressing... 264 gep_type_iterator I = gep_type_begin(CE), E = gep_type_end(CE); 265 for (++I; I != E; ++I) 266 if (const StructType *STy = dyn_cast<StructType>(*I)) { 267 ConstantInt *CU = cast<ConstantInt>(I.getOperand()); 268 assert(CU->getZExtValue() < STy->getNumElements() && 269 "Struct index out of range!"); 270 unsigned El = (unsigned)CU->getZExtValue(); 271 if (ConstantStruct *CS = dyn_cast<ConstantStruct>(C)) { 272 C = CS->getOperand(El); 273 } else if (isa<ConstantAggregateZero>(C)) { 274 C = Constant::getNullValue(STy->getElementType(El)); 275 } else if (isa<UndefValue>(C)) { 276 C = UndefValue::get(STy->getElementType(El)); 277 } else { 278 return 0; 279 } 280 } else if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) { 281 if (const ArrayType *ATy = dyn_cast<ArrayType>(*I)) { 282 if (CI->getZExtValue() >= ATy->getNumElements()) 283 return 0; 284 if (ConstantArray *CA = dyn_cast<ConstantArray>(C)) 285 C = CA->getOperand(CI->getZExtValue()); 286 else if (isa<ConstantAggregateZero>(C)) 287 C = Constant::getNullValue(ATy->getElementType()); 288 else if (isa<UndefValue>(C)) 289 C = UndefValue::get(ATy->getElementType()); 290 else 291 return 0; 292 } else if (const PackedType *PTy = dyn_cast<PackedType>(*I)) { 293 if (CI->getZExtValue() >= PTy->getNumElements()) 294 return 0; 295 if (ConstantPacked *CP = dyn_cast<ConstantPacked>(C)) 296 C = CP->getOperand(CI->getZExtValue()); 297 else if (isa<ConstantAggregateZero>(C)) 298 C = Constant::getNullValue(PTy->getElementType()); 299 else if (isa<UndefValue>(C)) 300 C = UndefValue::get(PTy->getElementType()); 301 else 302 return 0; 303 } else { 304 return 0; 305 } 306 } else { 307 return 0; 308 } 309 return C; 310} 311 312 313//===----------------------------------------------------------------------===// 314// Constant Folding for Calls 315// 316 317/// canConstantFoldCallTo - Return true if its even possible to fold a call to 318/// the specified function. 319bool 320llvm::canConstantFoldCallTo(Function *F) { 321 const std::string &Name = F->getName(); 322 323 switch (F->getIntrinsicID()) { 324 case Intrinsic::sqrt_f32: 325 case Intrinsic::sqrt_f64: 326 case Intrinsic::bswap_i16: 327 case Intrinsic::bswap_i32: 328 case Intrinsic::bswap_i64: 329 case Intrinsic::powi_f32: 330 case Intrinsic::powi_f64: 331 // FIXME: these should be constant folded as well 332 //case Intrinsic::ctpop_i8: 333 //case Intrinsic::ctpop_i16: 334 //case Intrinsic::ctpop_i32: 335 //case Intrinsic::ctpop_i64: 336 //case Intrinsic::ctlz_i8: 337 //case Intrinsic::ctlz_i16: 338 //case Intrinsic::ctlz_i32: 339 //case Intrinsic::ctlz_i64: 340 //case Intrinsic::cttz_i8: 341 //case Intrinsic::cttz_i16: 342 //case Intrinsic::cttz_i32: 343 //case Intrinsic::cttz_i64: 344 return true; 345 default: break; 346 } 347 348 switch (Name[0]) 349 { 350 case 'a': 351 return Name == "acos" || Name == "asin" || Name == "atan" || 352 Name == "atan2"; 353 case 'c': 354 return Name == "ceil" || Name == "cos" || Name == "cosf" || 355 Name == "cosh"; 356 case 'e': 357 return Name == "exp"; 358 case 'f': 359 return Name == "fabs" || Name == "fmod" || Name == "floor"; 360 case 'l': 361 return Name == "log" || Name == "log10"; 362 case 'p': 363 return Name == "pow"; 364 case 's': 365 return Name == "sin" || Name == "sinh" || 366 Name == "sqrt" || Name == "sqrtf"; 367 case 't': 368 return Name == "tan" || Name == "tanh"; 369 default: 370 return false; 371 } 372} 373 374static Constant *ConstantFoldFP(double (*NativeFP)(double), double V, 375 const Type *Ty) { 376 errno = 0; 377 V = NativeFP(V); 378 if (errno == 0) 379 return ConstantFP::get(Ty, V); 380 errno = 0; 381 return 0; 382} 383 384/// ConstantFoldCall - Attempt to constant fold a call to the specified function 385/// with the specified arguments, returning null if unsuccessful. 386Constant * 387llvm::ConstantFoldCall(Function *F, Constant** Operands, unsigned NumOperands) { 388 const std::string &Name = F->getName(); 389 const Type *Ty = F->getReturnType(); 390 391 if (NumOperands == 1) { 392 if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) { 393 double V = Op->getValue(); 394 switch (Name[0]) 395 { 396 case 'a': 397 if (Name == "acos") 398 return ConstantFoldFP(acos, V, Ty); 399 else if (Name == "asin") 400 return ConstantFoldFP(asin, V, Ty); 401 else if (Name == "atan") 402 return ConstantFP::get(Ty, atan(V)); 403 break; 404 case 'c': 405 if (Name == "ceil") 406 return ConstantFoldFP(ceil, V, Ty); 407 else if (Name == "cos") 408 return ConstantFP::get(Ty, cos(V)); 409 else if (Name == "cosh") 410 return ConstantFP::get(Ty, cosh(V)); 411 break; 412 case 'e': 413 if (Name == "exp") 414 return ConstantFP::get(Ty, exp(V)); 415 break; 416 case 'f': 417 if (Name == "fabs") 418 return ConstantFP::get(Ty, fabs(V)); 419 else if (Name == "floor") 420 return ConstantFoldFP(floor, V, Ty); 421 break; 422 case 'l': 423 if (Name == "log" && V > 0) 424 return ConstantFP::get(Ty, log(V)); 425 else if (Name == "log10" && V > 0) 426 return ConstantFoldFP(log10, V, Ty); 427 else if (Name == "llvm.sqrt.f32" || Name == "llvm.sqrt.f64") { 428 if (V >= -0.0) 429 return ConstantFP::get(Ty, sqrt(V)); 430 else // Undefined 431 return ConstantFP::get(Ty, 0.0); 432 } 433 break; 434 case 's': 435 if (Name == "sin") 436 return ConstantFP::get(Ty, sin(V)); 437 else if (Name == "sinh") 438 return ConstantFP::get(Ty, sinh(V)); 439 else if (Name == "sqrt" && V >= 0) 440 return ConstantFP::get(Ty, sqrt(V)); 441 else if (Name == "sqrtf" && V >= 0) 442 return ConstantFP::get(Ty, sqrt((float)V)); 443 break; 444 case 't': 445 if (Name == "tan") 446 return ConstantFP::get(Ty, tan(V)); 447 else if (Name == "tanh") 448 return ConstantFP::get(Ty, tanh(V)); 449 break; 450 default: 451 break; 452 } 453 } else if (ConstantInt *Op = dyn_cast<ConstantInt>(Operands[0])) { 454 uint64_t V = Op->getZExtValue(); 455 if (Name == "llvm.bswap.i16") 456 return ConstantInt::get(Ty, ByteSwap_16(V)); 457 else if (Name == "llvm.bswap.i32") 458 return ConstantInt::get(Ty, ByteSwap_32(V)); 459 else if (Name == "llvm.bswap.i64") 460 return ConstantInt::get(Ty, ByteSwap_64(V)); 461 } 462 } else if (NumOperands == 2) { 463 if (ConstantFP *Op1 = dyn_cast<ConstantFP>(Operands[0])) { 464 double Op1V = Op1->getValue(); 465 if (ConstantFP *Op2 = dyn_cast<ConstantFP>(Operands[1])) { 466 double Op2V = Op2->getValue(); 467 468 if (Name == "pow") { 469 errno = 0; 470 double V = pow(Op1V, Op2V); 471 if (errno == 0) 472 return ConstantFP::get(Ty, V); 473 } else if (Name == "fmod") { 474 errno = 0; 475 double V = fmod(Op1V, Op2V); 476 if (errno == 0) 477 return ConstantFP::get(Ty, V); 478 } else if (Name == "atan2") { 479 return ConstantFP::get(Ty, atan2(Op1V,Op2V)); 480 } 481 } else if (ConstantInt *Op2C = dyn_cast<ConstantInt>(Operands[1])) { 482 if (Name == "llvm.powi.f32") { 483 return ConstantFP::get(Ty, std::pow((float)Op1V, 484 (int)Op2C->getZExtValue())); 485 } else if (Name == "llvm.powi.f64") { 486 return ConstantFP::get(Ty, std::pow((double)Op1V, 487 (int)Op2C->getZExtValue())); 488 } 489 } 490 } 491 } 492 return 0; 493} 494 495